1. Field of the Invention
This invention relates to the structures of driving and driven sprockets and belt reception rotary member in a paper feed apparatus that are supported rotatably between a pair of side frames for passing a feed belt around them.
2. Description of the Prior Art
Driving of a feed belt in a paper feed apparatus is by sprockets.
A feed belt is passed around a driving sprocket disposed between a pair of side frames disposed facing each other with a predetermined gap between them and supported at one of the end portions of these side frames, and a receiving member disposed at the other portion of the side frames, and this feed belt is driven by the rotation of the driving sprocket. There are cases where this receiving member is a belt receiver disposed integrally with the inner side surfaces of the pair of side frames, where it is a short cylindrical belt reception rotary member supported rotatably between the pair of side frames, and where it is a similar driven sprocket.
The structure of a known driving sprocket and the structure for supporting this driving sprocket in a pair of side frames are shown in FIGS. 17-20.
In this driving sprocket 51, engaging portions 53 project from both end surfaces of a sprocket main body 52 and these portions 53 are fitted into receiving holes 55 disposed in a pair of side frames 54a, 54b so that the driving sprocket 51 is supported rotatably by the pair of side frames 54a, 54b. A square shaft hole 56 is bored in the driving sprocket 51, a square driving shaft 57 is fitted into this shaft hole 56, and the driving sprocket 51 is rotated by the rotation of the driving shaft 57.
Quite naturally, the distance A' between the opposed surfaces of the driving shaft 57 is smaller than the distance B' between the opposed inner peripheral surfaces of the shaft hole 56 and in the state where the driving shaft 57 is inserted into the shaft hole 56, a predetermined clearance exists between them. Therefore, as shown in FIG. 19, the driving sprocket 51 can move a small amount with respect to the driving shaft 57 in its rotating direction and a phase difference .theta. occurs between the rotation of the driving shaft 57 and the driving sprocket 51 during the paper feed operation. This phase difference .theta. lowers paper feed accuracy and, eventually, the printing accuracy of a printer deteriorates. Then, as shown in FIGS. 19 and 20, when the driving shaft 57 is rotated in both forward and reverse directions to feed a sheet of paper in these directions, the phase difference described above becomes 2.theta. so that the influences of the phase difference on the rotation of each of the driving sprockets 51 and the driving shaft 57 becomes larger.
This problem can be eliminated by minimizing the clearance between the shaft hole 56 of the driving sprocket 51 and the driving shaft 57. However, since the conventional driving sprocket 51 has a structure wherein the fitting portions 53 protrude from both end surfaces of the sprocket main body 52 as described above, the length L' of the shaft hole 56 becomes large and the thickness of the peripheral portion of the shaft hole 56 differs greatly in its longitudinal direction. In the case of a driving sprocket 51 consisting of a resin molded article, therefore, there is a problem that it is difficult from the aspect of molding to make the dimension of the shaft hole 56 uniform in the longitudinal direction. Accordingly, a design which makes it possible to smoothly insert the driving shaft 57 in the smallest portion of the shaft hole 56 must be employed so that the clearance between the shaft hole 56 and the driving shaft 57 becomes larger and this results in the reduction of paper feed accuracy.
As a paper feed apparatus which improves paper feed accuracy during the paper feed operation by preventing the fluctuation of the center distance between the driving sprocket supported at one of the end portions of the pair of side frames and the receiving member disposed at the other end portion, there is known a structure wherein auxiliary frames are fitted to the inner side surfaces of the pair of side frames in such a manner as to be capable of moving slightly in the travelling direction of the feed belt, the driving sprocket is supported rotatably by the auxiliary frames and the receiving member is provided on them. In the paper feed apparatus having such a structure, the thickness of the auxiliary frames cannot be made so great that the driving sprocket 51 having the structure wherein the fitting portions 53 protrude from both side surfaces of the sprocket main body 52 such as described above cannot be supported.
In such a paper feed apparatus equipped with the auxiliary frames, if the driven sprocket or belt reception rotary member has the same structure as that of the driving sprocket 51 when such a driving sprocket or belt reception rotary member is used as the receiving member for the feed belt, it cannot be supported by the auxiliary frames for the same reason as described above.